Results of light-scattering experiments on helium and xenon gas mixtures are presented for a large range of wave vectors, densities, and compositions. In the experiments these variables are varied independently from each other. In this manner we are able to show that the hydrodynamic eigenfrequencies, reduced with respect to the product of the wave vector k and the adiabatic sound velocity cₛ, are a function of the product of the wave vector and the mean free path lHe only. By comparing the experimentally obtained dispersion curves and the predictions of the hydrodynamic theory, we find that the value of klHe at which the theory ceases to be valid is independent of the composition. We show that the longitudinal current-current correlation function ω²I(k,{ω}) is a useful function in the study of light-scattering spectra that are featureless. For large reduced wave vectors we find a sound mode that is solely supported by the fluctuations in the xenon density. This is the slow sound mode.
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Wegdam et al. (1989) studied this question.